1. Field of the Invention
The present invention relates to an X-ray source of the type having an electron emitter for the production of an electron beam, an anode on which the electron beam strikes in an X-ray focal spot, and a magnet system that produces a dipole field and a quadrupole field superimposed thereon, for the deflection and focusing of the electron beam onto the anode.
2. Description of the Prior Art
In an X-ray source known from U.S. Pat. No. 5,883,936, fashioned as a rotating bulb source, a magnet system is provided for the deflection and focusing of the electron beam, which emanates from the electron emitter and has an initially circular cross-section. For the production of an X-ray focal spot that is substantially circular, seen opposite the primary direction of propagation of the X-rays emanating from the X-ray focal spot, the quadrupole field is selected such that it modifies the cross-section of the electron beam emitted by the cathode, which initially has a circular cross-section. This modification occurs in such a way that the X-ray focal spot arising at the edge of the anode is elongated in the radial direction due to the anode edge being beveled relative to the primary direction of radiation of the X-ray radiation, relative to the width of the electron beam measured in the tangential direction (length-to-width ratio). This has in turn the result that, as seen in the direction opposite the primary direction of propagation of the X-rays emanating from the X-ray focal spot, the extension of the electron beam in the radial direction corresponds to the extension of the electron beam in the tangential direction. The X-ray focal spot thus has substantially circular shape, with the electron density of the electron beam shortly before the X-ray focal spot being higher than immediately adjacent to the cathode. The Gaussian distribution of the electrons over the cross-section is, however, maintained.
Expensive measures would be necessary in order to enable an adjustment of the size of the X-ray focal spot in such a rotating bulb source, e.g. by means of a switching the largest elongation.
The size of the focal spot could be adjusted in a known manner by means of an adjustable focusing voltage applied to a focus cup that surrounds the electron emitter. An electron emitter with a variable emission surface alternatively could be used that could be constructed as a flat or spiral emitter with several emission surfaces, in particular concentrically arranged, which can be activated individually or together, corresponding to the desired size of the X-ray focal spot. This would have the advantage that the type of drive would be maximally compatible with existing generators. However, disadvantages would include higher manufacturing costs and reduced flexibility. In addition, narrow tolerances in the cathode manufacturing would have to be taken into account.
In addition, it is disadvantageous that neither of the two possibilities offers advantageous conditions for an optimization of the intensity distribution of the X-rays emanating from the X-ray focal spot in the sense of a rectangular curve of the intensity of the X-rays in the radial direction of the X-ray focal spot.